This invention relates to an anti-locking brake system in which the pressure acting on the wheel brakes is reducible by bleeding hydraulic fluid through a return line utilizing a hydraulic pump whose suction chamber is connected through a suction line to a hydraulic fluid reservoir and is an improvement of the system disclosed in published German patent application No. 37 40 516.0.
A brake system of this type is known from published German patent application No. 36 01 914. In that brake system, the return line connects the outlet valves hydraulically to the hydraulic fluid reservoir. Since the pump operates only in case of need, the full pump capacity must be instantaneously available and, thus, a high pumping efficiency is of vital importance. In particular, in the case of reciprocating piston pumps with suction and discharge valves, the suction behavior encounters problems at very low temperatures (i.e., when the fluid has a high viscosity). If the auxiliary pressure supply system fails (for example, because the pump fails to start or fails to deliver), then the outlet valves must be prevented from opening so that a sufficient quantity of hydraulic fluid remains in the master cylinder or wheel cylinder in order to be able to at least ensure uncontrolled braking without having to employ the auxiliary pressure. For this reason, intricate and expensive measures have been necessary to monitor the brake system or the pressure supply and to rapidly detect any malfunctions as well as to partly or entirely switch off the control.